Cyanide is an extremely toxic chemical. It is used in many industries, including metallurgy, automobile manufacturing, and wood working. Specifically, cyanide is a critical chemical for use in electroplating, coal conversion, and iron ore separation. Various cyanide salts are produced as by-products in these reactions, including sodium cyanide, potassium cyanide, and calcium cyanide.
The waste material generated from such processes is extremely dangerous due to the presence of cyanide and cyanide salts. Consequently, the storage, transport, and disposal of this cyanide waste presents serious hazards to persons handling the waste and to the environment.
Existing methods for treating cyanide waste include wet oxidation and chemical or heat treatment. For example, Brandenburg et al. (U.S. Pat. No. 4,812,243) discloses a method for operating a continuous wet oxidation system for destroying cyanide in caustic cyanide and metal wastes which scale upon heating.
Dvorscek et al. (U.S. Pat. No. 4,802,995) discloses a method of decomposing the cyanide radical in cyanide-bearing wastes using heat treatment. The method is performed by introducing cyanide-bearing waste and water into a closed retort and heating the retort above a critical temperature.
Robey (U.S. Pat. No. 4,877,519) discloses a method for treating cyanide-containing waste water using a sufficiently high temperature and pressure to affect hydrolysis of cyanide in the waste water. The disclosed method utilizes a double heat exchange system for both influent and effluent streams of waste water.
Fischer et al. (U.S. Pat. No. 4,731,232) discloses a method for purifying gas wash waters containing hydrogen cyanide. The method involves controlled dosing of the waste with aqueous formaldehyde.
Other methods for treating cyanide waste have been developed utilizing biological material. For example, Timofeeva et al. (Soviet Union Patent 1468871) discloses degradation of cyanides in cyanide-containing waste using Pleurotus ostreatus fungi. The cyanide-degrading fungi are immobilized on lignin sludge or on a mixture of sawdust and aerosil and then exposed to the waste.
Kubota (Japanese Patent 1293195) discloses a method of treating cyanide-containing waste water using the bacteria Bacillus subtillis Kabata. Kubota reports that the bacteria may be added directly to the waste water or aggregated on a support such as active carbon or glass beads and then added to the waste.
Castaldi et al. (U.S. Pat. No 4,790,940) discloses a method of detoxifying cyanide-containing effluent involving a combination of chemical and biological treatment. The process disclosed employs chemical conversion with polysulfide to transform the free cyanide in the effluent to thiocyanate. The thiocyanates are then metabolized by the action of certain gram-negative chemolithotropic bacteria.
Some of the aforementioned methods are inadequate in detoxifying cyanide to safe levels and produce waste which is environmentally dangerous. Furthermore, some of the methods described above require special equipment and are labor intensive.